The Threshold of Serum Anion Gap as a Screening Tool for Organic Acidosis

Background: The serum anion gap (AG) is commonly used as a screening tool for acid-base disorders. With modern laboratory techniques using ion-selective electrodes to measure the main electrolyte components of the AG, our definition high AG (HAGMA) should be reviewed.

Aim: This study aims to assess the diagnostic value of AG and to determine a diagnostic threshold for HAGMA in a high-prevalence clinical setting.

Method: Computerized extraction of anonymised data from electronic medical records was performed. A pre-defined criteria included all inpatients of an acute-care hospital who had measurements for organic acids (lactate, ketone or salicylate) paired with a serum urea, electrolyte and creatinine panel.

Study Overview

• Status: Completed
• Conditions: Critical Illness; Metabolic Acidosis; Anion Gap Acidosis; Lactic Acidosis; Keto Acidosis; Acid Base Disorder

Detailed Description

BACKGROUND: The urea & electrolyte (U&E) panel is a common blood test that is often performed for any ill patients in a hospital. Evaluation of this panel can reveal serious underlying acid-base disturbances, often a harbinger of further haemodynamic compromise. The detection of a gap acidosis often is the first indicator of a serious underlying condition.

The AG is a reflection of the unmeasured anions. The anion gap (AG) is derived from the U&E panel by subtracting measured anions from measured cations:

AG = [Sodium] + [Potassium] - [Chloride] - [Bicarbonate] The serum potassium can be included in the calculation, but due to low variability of serum potassium, most clinicians omit serum potassium in the calculation of AG.

Accumulation of organic acids (e.g. lactic acid, ketones, uremic toxins) results in a high AG metabolic acidosis (HAGMA) and such patients have a significant risk of further clinical deterioration. It has however been challenging to assess for gap acidosis as the range for normal AG can vary greatly due to historical differences in the laboratory techniques used to measure the constituents of AG as well as a component of interindividual variability. In the past, AG were reported (in milliequivalents per liter, mEq/L) in the range of 9-17 milliequivalents/L or 8-16 milliequivalents/L with older laboratory techniques such as flame photometry, colorimetric method. In more recent years with the advent of use of ion-selective electrodes (ISE), published literature suggests that the AG range may be lower as the ISE may give higher readings of serum chloride. Some clinical texts and laboratories have listed the reference range of AG as 3-11 milliequivalents/L or 5-12 milliequivalents/L.

Despite so, the reference range for serum sodium, potassium and chloride themselves have remained the same over the years despite changes in laboratory techniques. With such conflicting information, it is important to re-evaluate the screening or diagnostic threshold of the AG based on modern laboratory techniques.

OBJECTIVES: To assess the diagnostic threshold of AG to detect gap acidosis due to organic acidosis.

METHODS: Study protocol was reviewed and approved by the institutional ethics committee (SingHealth centralised institutional review board: 2019/2463). A waiver of consent was obtained as data was anonymised.

This is a retrospective study. Data will be extracted from electronic medical and laboratory records from all inpatient admissions to an acute hospital. Data extracted will be anonymized.

The search criteria will include the following parameters:

• all inpatient admissions 2017 - 2020 and
• patients with serum lactate, ketones and salicylates performed, and
• patients with urea, electrolytes and creatinine at the same time.

Data to be extracted include baseline demographic data, admission date, and laboratory results including urea / electrolytes / creatinine, albumin, glucose, calcium, phosphate, magnesium, albumin, liver function test, arterial blood test and hemoglobin.

If patients had multiple blood samples taken, only laboratory results from a single sample will be included. This will be selected by an automated algorithm which selects the sample with the most complete information.

Statistical Analysis Plan: The patients will be divided into 2 groups - with / without organic acidosis (elevated serum lactate, ketones and/or salicylates). An area under the curve (AUC) of receiver operating characteristics (ROC) analysis of AG for the detection of organic acidosis will be performed. The optimal cut-off threshold will be explored using the Youden index. Analysis of the diagnostic threshold of AG at different cut-off levels will be performed and the performance of the AG as a screening test at different cut-off thresholds will be assessed by reviewing the sensitivity, specificity, positive/negative predictive values and related indicators. A recommended cut-off value will be determined based on a test sensitivity adjudicated to be acceptable.

• Study Type: Observational
• Enrollment (Actual): 16475

Contacts and Locations

Study Locations

• Singapore
  • Singapore, Singapore, 529889
    • Changi General Hospital

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Child; Adult; Older Adult
• Accepts Healthy Volunteers: No

• Sampling Method: Non-Probability Sample

Study Population

Inpatients in acute care hospital admitted from 01 July 2017 to 31 December 2019 who were being assessed for organic acidosis

Description

Inclusion Criteria:

• Admitted from 01 July 2017 to 31 December 2019
• Patients who have had blood lactate, blood ketones and serum salicylates performed during this time period
• Patient who also had urea / creatinine and a comprehensive panel of electrolytes performed up to +/-12h of blood lactate, blood ketones and serum salicylates being performed

Exclusion Criteria:

• Patient who did not have blood lactate, blood ketones and serum salicylates performed during this time period
• Patient who did not have urea / creatinine and a comprehensive panel of electrolytes performed up to +/-12h of blood lactate, blood ketones and serum salicylates being performed

Study Plan

How is the study designed?

Design Details

• Observational Models: Cohort
• Time Perspectives: Retrospective

Number of groups / cohorts

1

Cohorts and Interventions

Group / Cohort
Participants; All inpatient admissions with serum lactate, ketones and/or salicylates performed, and with urea, electrolytes and creatinine at the same time. The patients will be grouped into patients with organic (gap) acidosis (elevated serum lactate, ketones and/or salicylates) and patients with no organic acidosis.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Area Under the Curve of Receiver Operating Characteristics	An area under the curve (AUC) of receiver operating characteristics (ROC) of Anion Gap for the presence of organic acidosis will be calculated. An AUC >0.8 suggests that Anion Gap performs well as a test for the presence of organic acidosis.	End of study period, estimated 3 years
An optimal cut-off threshold of Anion Gap	The optimal cut-off threshold will be determined. At each cut-off level. the sensitivity and specificity will be calculated. One recommended cut-off value for Anion Gap with a sensitivity >95% will be accepted.	End of study period, estimated 3 years

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Subgroup Analysis for Performance of Anion Gap as a screening test for Organic Acidosis	The AUC-ROC for Anion Gap will be calculated for each patient subgroup: serum urea (≤7.7 mmol/L or >7.7 mmol/L), age (below or above median age), sex, race and concomitant respiratory acid-base disorders (present or absent)	End of study period, estimated 3 years

Collaborators and Investigators

• Sponsor: Changi General Hospital
• Investigators: Principal Investigator: Chang Y Chionh, Changi General Hospital

Publications and helpful links

General Publications

• Winter SD, Pearson JR, Gabow PA, Schultz AL, Lepoff RB. The fall of the serum anion gap. Arch Intern Med. 1990 Feb;150(2):311-3.
• Sadjadi SA, Manalo R, Jaipaul N, McMillan J. Ion-selective electrode and anion gap range: What should the anion gap be? Int J Nephrol Renovasc Dis. 2013 Jun 7;6:101-5. doi: 10.2147/IJNRD.S44689. Print 2013.
• Ayala-Lopez N, Harb R. Interpreting Anion Gap Values in Adult and Pediatric Patients: Examining the Reference Interval. J Appl Lab Med. 2020 Jan 1;5(1):126-135. doi: 10.1373/jalm.2019.029496.
• Lolekha PH, Vanavanan S, Lolekha S. Update on value of the anion gap in clinical diagnosis and laboratory evaluation. Clin Chim Acta. 2001 May;307(1-2):33-6. doi: 10.1016/s0009-8981(01)00459-4.

Study record dates

Study Major Dates

• Study Start (Actual): July 1, 2017
• Primary Completion (Actual): December 31, 2021
• Study Completion (Actual): June 6, 2023

Study Registration Dates

• First Submitted: May 28, 2021
• First Submitted That Met QC Criteria: July 23, 2021
• First Posted (Actual): July 26, 2021

Study Record Updates

• Last Update Posted (Actual): June 15, 2023
• Last Update Submitted That Met QC Criteria: June 14, 2023
• Last Verified: June 1, 2023

More Information

Terms related to this study

• Keywords: Screening Test; Anion Gap; Metabolic Acidosis; Lactic Acidosis; Ketoacidosis
• Additional Relevant MeSH Terms: Pathologic Processes; Metabolic Diseases; Disease Attributes; Acid-Base Imbalance; Critical Illness; Acidosis; Acidosis, Lactic; Ketosis
• Other Study ID Numbers: SingHealth CIRB: 2019/2463

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: NO

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No